TRANSACTIONS
IEEE SONICS
ON
AND ULTRASONICS, VOL. SU-23, NO. 5 , SEPTEMBER 329
1976
Evaluation of a Random Signal Correlation System
for Ultrasonic F l a w Detection Abstract-A random signal flaw detection system has been developed with a signal-to-noiseratio enhancement approximately10 000
times greater than conventional ultrasonic pulse-echo systems.Experimental results are presented which demonstrate thatthe system
can penetrate highly absorbentmaterials such as plexiglass,highdensity siliconnitride and alumina at greater distances than is possible
with conventional pulse-echo systems. The systemis shown to detect
echoes fromgrain boundarieswhich indicate that the system could
have applications in microstructure evaluationof materials. Finally,
the possible applicationof this system to accurate sound velocity measurement in strongly absorbing materialsis discussed.
INTRODUCTION
LTRASONIC pulse-echo flaw detection systems have
been widely used in commercial applications as a quality control device in detecting flaws and defects in materials
ranging from metals to ceramics. However, these systems are
governed by the following relationship:
U
max range
range resolution
interval < burst
burst width
peak power
average power
.
(1)
Therefore, the ratio of the maximum range t o the desired
range resolution is limited by the peak power the transducers
can handle without electrical breakdown. Another inherent
problem of pulse-echo flaw detectors is that strongly sound
absorbing materials require the use of the largest possible average transmitted power if the returning echoesare to be larger
than the thermal receiver noise [ l ] . However, since the peak
power is limited by the transducer, the average power can only
be increased at the expense of either range or resolution.
The random signal flaw detection system [2] which is evaluated in this paper overcomes these important limitations inherent in conventional pulse-echo systems. By utilizing correlation and time integration techniques, the randomsignal
system obtains a signal-to-noiseratio enhancement of approximately 10 000. In addition, this system uses noise as the
transmitted signal so that the resolution along the ultrasonic
beam is independent of the signal duration. Consequently,
the peak-to-average transmitted power ratiocan be kept close
to unity, so that the maximumpower that can be transmitted
is no longer limited by transducer peak power breakdown.
These advantages allow the random signal system to be used
on morehighly absorbent materials and to detectsmaller
Manuscript received April 19, 1976. This work was supportedin
part by the Advanced Research Projects Agency and in part by the
Purdue University NSF-MRL Program under Contract DMR
7203018-AO4.
The authorsare with the School of ElectricalEngineering, Purdue
University, Lafayette,IN 47907.
flaws at greater distances than is possible with pulse-echo
systems.
Time integration as a means of increasing sensitivity has
previously been used with pulsed radio frequency ultrasound
signals in time averaging systems. We have compared the
performance of pulsed rf and random signal correlation systems [3] with that of signal averaging systems which operate
in the frequency [4] and time domain [S] . The conclusion
reached is that the randomsignal system perfoms equally
well as the time averagers and can be superior if clutter is not
the dominant form of system noise.
The remainder of the paper presentsa description of the
system and discusses the experimental results obtained.
SYSTEM
DESCRIPTION
A block diagram of the random signal flaw detection system
is shown in Fig. 1. The electrical signals produced by the
noise source are converted to ultrasound and transmitted into
the sample by the piezoelectric transducer. Echoes reflected
from inhomogeneities are picked up by the same transducer
and are reconverted into electrical signals. The amplified
echo signal and the reference signal emerging from the delay
line are fed through Schmitt triggers acting as clipping circuits. The clipped signals are then passed to a digital correlator whose gated output is displayed on t h e y axis of a pen
recorder. The x axis of the recorder corresponds t o the
separation between thedelay line transducers.
To scan a portion of a specimen, theseparation between
the delay line transducers is vaned. Whenever the reference
signal delay ~d approximately equals the time of flight rs,
the correlator producesan output which is displayed on the
pen recorder. For a single-surface reflector, the output of the
system is the autocorrelation fucntion Rx(rs - r d ) of the
transmitted signal x ( t ) .
TRANSMJCW
Fig. 1. Experimentalrandomsignal flawdetectionsystememploying
signal clipping and polarity coincidence correlator.
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IEEE TRANSACTIONS ON SONICS AND
ULTRASONICS, SEPTEMBER
1916
the random signal system is given by the equation
where B is the bandwidth of the transmitted noise, T i s the integration time of the correlator,(Y is the on-off ratio of the
transmitted signal, and 2/n is a correction factor dueto signal
clipping [6] .
EXPERIMENTAL RESULTS
An earlier version of this system using separate transducers
for transmission and reception was operated successfully in
detecting a variety of wire targets in water. These results,
which were published in an earlier paper [2], verify that the
resolution of the randomsignal system is independent of signal
m> duration
and show that thesignal-to-noise ratio enhancement is
Fig. 2. (a) Spectrum of the transmitted noise signal. (b) Correlator
given by (5).
output voltage as range cell is scanned through target.
In the remainder of this section we present results obtained
for the current single transducer system which was developed
by modifying the double transducersystem to simplify the
Fig. 2 shows the system output for a bell-shaped transmitted
examination of samples. These results include detection of
noise spectrum
drill holes in ceramics and plastics, detection of interfacesbetween metals, and examination of samples with varying grain
sizes.
To demonstrate theapplication of the random signal system
where fo is the center frequency andB is the bandwidth of the
to highly absorbing materials, we present some results on
transmitted signal. The correlator output is inverse
the
Fourier
samples such as plexiglass, alumina, andhigh-density silicon
transform of S, (f) and may be written as
nitride. Figs. 3 and 4 show the outputs obtained for two
R x ( T s - Td)=B/2eXp {-nBITs- T d l }
plexiglass samples which have an absorption coefficient of
lOdB cm-' at 5 MHz [7] . In Fig. 3 the back surface of a
' cos {2nfo(Ts - Td)) .
(3)
13-cm long plexiglass rod is clearly visible, even though the received
echo was deeply buried in thermal receiver noise. Fig.
It can be seen that the correlator outputreaches a maximum
4
shows
the output for anotherplexiglass sample consisting of
when rs = Td and falls to l / e of its maximum when lrs - $-dl =
two
cylinders
fused together and containinga series of conl / r B . Thus it follows that the range resolution of this system
centric
flat-bottomed
holes ranging in diameter from to
is approximately
in. The high signal-to-noise ratio enhancement allows this
AR = c/nB
(4) system to recover the echoes received from both the joint and
drill holes. It is important to note that in both cases the
where c is the velocity of sound in the test object. Note that
echoes received from points of interestwere not visible above
in contrast to conventional pulse-echo orsignal-averaging systhe thermal receiver noise and therefore could not have been
tems using deterministic pulsed rf signals, the range resolution
detected by a conventional pulse-echo flaw detection system.
of the random signal system is independent of the transmitted
Ceramic materialssuch as alumina and Si3N4(silicon nitride)
signal length. Thus, in the absence of clutter, the transmitted
are also highly absorbent and therefore difficult to examine
signal bursts can be kept long, with a consequent reduction in
by a pulse echo system. The versatility of the random signal
the peak-to-average transmitted power ratio [2].
system in examining suchmaterials is shown in Figs. 5 and 6.
In Fig. 5 the correlator output fora 1 2 c m long piece of
SIGNAL-TO-NOISE
RATIO IMPROVEMENT
alumina rod shows clearly that the random signal system not
only detects the back surface of the sample but also the
An important property of the randomsignal flaw detection
grain echoes. Fig. 6 shows that the system can readily detect
system is that its sensitivity can be made arbitrarily large by
a 16-mil laser-bored hole in highdensity silicon nitride.
simply increasing the integration time of the correlator. Since
Figs. 7 and 8 show the results obtained for a pure copper
the input and output bandwidthsare comparable in convensample which was heat treated several times t o enhance the
tional pulse-echo systems, such systems cannot obtain signalgrain size. The outputs showclearly that the system is able
to-noise ratio enhancement. However, this is not the case for
to detect the growth in thegrain size resulting from consecthe random signal system since the bandwidth at thereceiver
utive heat treatments. A conventional pulse-echo flaw detecis determined by the transmitted bandwidth Bin while the
tion system would have failed to detect thegrains even after
output bandwidth Bout depends on thereciprocal of the integration time of the correlator which can be made arbitrarily
the heat treatmentssince the grain echoes remained buried in
long. Thus the signal-to-noise ratio improvement provided by
noise. Since the clipping circuits give identical outputs when
&
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BILGUTAY et al.: RANDOMSIGNALCORRELATIONSYSTEM
Fig. 3. Scan of 13cm long plexiglass rod.
Transducer
-,!ant
l
Fig. 4. (a) Plexiglass testblockcontainingconcentricflat-bottomed
holes. (b) Correlator output for plexiglasstestblock showing location of joint and driU holes.
FROM SLRFACE
Fig. 5 . Scan of 12cm long alumina rod.
331
332
IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, SEPTEMBER 1976
16MILHOLE
FRONT SURFACE
I
I
I
BACK SURFACE
Fig. 6. Detection of 16-mil laser-bored hole in block o f silicon nitride
(Si3Nd.
I
FRONT SURFKE
Fig. 7 . Output for copper sample (5-9 pure) after two separate heat treatments of an hour each at 700" F and 1900" F,
respectively (average gain size: 0.19 mm).
FRONT SURFACE
+GRAIN ECHOES-+
Fig. 8. Output for copper sample (5-9 pure) after final heat treatment of two hours at 1900" F (average grain size:
0.32 mm).
333
BILGUTAY e t al.: RANDOM SIGNAL CORRELATIONSYSTEM
f
Fig. 9. (a)6-insteelcube
with-8-mm thickcladding. (b) Correlator
block
and
output showing detection of interface between steel
cladding.
received signals are above a given size, the system stays linear
provided that the input signals are much smaller than thermal
receiver noise. However, when input signals are comparable in
size to the receiver noise, the system operates in a nonlinear
regime, thus enhancing the weaker echoes. This effect is
clearly seen in Fig. 8 where the grain echoes result in outputs
similar in size to the front surface of the sample.
These results suggest that the random signal system could
serve as a tool for nondestructively estimating microstructure
of materials even deep inside the sample. The best known
technique currently used for grain size estimation consists of
etching surfaces, which is a destructive procedure. Since
ultrasonic pulse-echo systems are found to be too insensitive
to detect echoes from grain boundaries, current ultrasonic
evaluation of microstructure consists of attenuation measurements. The disadvantage of this technique is that the microstructure is averaged over the entire region traversed by the
ultrasonic beam. Furthermore, attenuation measurements
depend on the quality of the transducer-to-material surface
contact, thus being very difficult to reproduce, especially for
higher frequencies. Therefore, we believe that the application
of the random signal flaw detection system to microstructure
estimation could prove superior to current techniques.
The system can also detect echoesreflected from the interface of two differentmaterials even when these echoes are
smaller than the thermal receiver noise. Fig. 9 shows the
correlator output fora 6-in steel cube of the type used in
nuclear reactor pressure vessels with -8-mm thick cladding
of stainless steel. The echo from the cladding which was
buried in thermal receiver noise is clearly visible on the correlator output obtained by looking through
6 inches of steel.
Accurate velocity measurements are presently obtained by
using multiple reflections which cannot be produced in thick
samples of highly absorbent materials. It can be shown that
with a sufficiently sensitive system, sound delay and thus
velocity can be measured to approximately one part in l o 4 ,
even in strongly absorbing media. Therefore, we believe that
the random signal system can serve as an accurate means of
measuring sound velocity especially in absorbent materials.
CONCLUSION
We have demonstrated a random signal flaw detection system which has a signal-to-noise ratio enhancement on the
order of 10 000. Samples consisting of strongly sound absorbing materials such as plexiglass, high-density silicon nitride, and alumina were presented, showing that the system is
capable of penetrating such materials to a greater depth than
is possible with conventional pulse-echo systems. The system
was also shown to successfully detect interfaces of cladding in
steel. Data presented on pure copper samples indicate that
the system could serve as a tool for estimating microstructure
of materials. Finally, we believe that the random signal system can have possible applications in accurate sound velocity
measurements in highly absorbing materials.
REFERENCES
[ l ] J . Krautkramerand H. Krautkramer, Ultrasonic TestingofMaterials, Springer,New York, 1969.
[2] E. S. Furgason, V. L. Newhouse, N. M . Bilgutay, and G. R.
Cooper, “Application of Random Signal Correlation Techniques
to Ultrasonic Flaw Detection,” Ultrasonics, Vol. 13, No. 1,
January 1975.
[3] V. L. Newhouse and E. S. Furgason, “Ultrasonic Correlation
Techniques,” Purdue Univ. Report TR-EE 75-4 1, November
1975.
[4] J. A . Seydel, “Improved Discontinuity Detection in Ceramic Material Using Computer-Aided Ultrasonic Non-Destructive Techniques,” Symp. Proc. 2nd Army Mat. Tech. Conference, November 1973.
[S] J . C. Kennedy and W. E. Woodmansee, “Signal Processing in NonDestructive Testing,” Boeing Publication SAOPI-F01 RB2, April
1973.
[ 6 ] V. L. Newhouse, E. S . Furgason, and N. M . Bilgutay, “Random
Signal Flaw Detection,”Proceedings of the 1974 IEEE Ultrasonics Symp.
[7] P. N. T. Wells, Physical Principlesof Ultrasonic Diagnosis, Academic Press, 1969.